Encoding and decoding method and system

ABSTRACT

In a method and apparatus for encoding a tag with an n-bit binary code one or more predetermined frequency sources are associated with the tag that produce known different respective characteristic frequencies, each of which is associated with a known unique position in the n-bit binary code,

FIELD OF THE INVENTION

This invention relates to a method and system for encoding and decodingthe identification of a tag.

BACKGROUND OF THE INVENTION

Automatic identification may be effected in different ways includingmany bar codes, identification tags and may be used to identify articlesand personnel.

Radio Frequency Identification (RFID) is an automatic identificationtechnology similar in application to bar code technology, but uses radiofrequency instead of optical signals. An RFID system consists of twomajor components—a reader and a tag or card. They work together toprovide the end user with a non-contact solution to uniquely identifypeople, animals or objects. The reader performs several functions, oneof which is to produce a low-level radio frequency magnetic field thatserves as a “carrier” of power from the reader to the RFID tag.

A passive RFID tag contains an antenna and an integrated circuit (IC).The IC requires only a minimal amount of electrical power to function.The antenna in the tag provides a means for gathering the energy presentin the magnetic field produced by the reader, and converts it to anelectrical signal for use by the IC. When a tag is brought into themagnetic field produced by the reader, the recovered energy powers theIC, and an electromagnetic signal modulated with data in the memory istransmitted by the tag's antenna. The electromagnetic signal transmittedfrom the tag is recovered by an antenna within the reader, and convertedback into an electrical form. The reader contains a sensitive receivingsystem that is designed to detect and process the weak tag signal,demodulating the original data stored in the tag memory.

As opposed to passive tags, active tags contain a miniature battery thatprovides the operating power for the IC. When interrogated by thereader, the IC broadcasts a signal that identifies itself to sensitivereader detection and data transmission circuits. This allows the tag tobegin sending its data at a considerably greater distance from thereader than its passive counterpart. Additionally, an active tag usesbattery energy to produce a much stronger electromagnetic responsesignal. All of this results in a significantly greater read range than apassive tag.

Many present and upcoming applications require the ability toautomatically identify objects from a distance. RFID provides thisrequirement to some degree and has the benefit that line-of-sightbetween the reader and tag is not required. This is distinct fromoptical recognition techniques, such as barcode, that require theoptical reader to be in line of sight from the identified object. Mostoptical reading techniques also require a directional decoding of theidentifying code, giving rise to start and stop bits being part of thecode. In a conventional, linear bar code, the order in which the barsappear is critical and changing the order results in a different code.The same is applicable also to WID tags, where typically reflectors inthe tag are used to modulate the RF signal, such that the presence orabsence of reflectors affects the resulting code that is returned by thetag.

U.S. Pat. No. 5,381,137 (“RFtagging system and RF tags and method”,assigned to Motorola, Inc., Schaumburg, Ill., published on 1995)discloses an RF tagging system that has a plurality of resonant circuitson a tag. When the tag enters a detection zone, the system determinesthe resonant frequency of each of the resonant circuits and produces acorresponding code. Resonant frequency detection is implemented bysimultaneously radiating signals at each of the possible resonantfrequencies for the tag circuits. The system is useful for coding anyarticles such as baggage or production inventory.

The reader in U.S. Pat. No. 5,381,137 comprises an antenna array havinga plurality of fixed location multiple transmitter frequency probes.Tags comprise a plurality of passive resonant circuits, each of whichmay resonate at any different frequency selected from a predeterminedplurality of known resonant frequencies. Each of the resonant circuitsis fixed at a different location on a planar surface of the tag. The jagis positioned between guide rails so as to fix its position with respectto the plurality of fixed location probes. Either the tag is moved suchthat various rows of tuned circuits pass directly under the probes, orthe probes are otherwise positioned directly above and in registrationwith the tuned circuits. By such means, each of the probessimultaneously radiates each of the possible resonant frequency signalsthat may correspond to the resonant frequency of any of the circuits.

Such a system is shown schematically in FIG. 1, showing a tag 10 that isencoded with multiple frequency sources to generate a code that isidentified by a reader 11. Thus, as shown, the tag 10 comprises onlythree different frequency sources, depicted respectively f₁ f₂ f₃. It isseen that each of these frequency sources may be used more than once,its presence in, or absence from, the tag being depicted by “Y” and “N”,respectively. This does not cause any ambiguity since the frequencysources are spatially separated and the reader 11 is adapted to readeach frequency source (which may be ambiguous) in association with itslocation (which is always unique). Thus, when a frequency sourcetransmits a signal to the reader 11, this indicates that the frequencysource is present at a known location, and the corresponding location inthe resulting code construed by the reader may be set to logic “1” (orto logic “0” if negative logic is used). Since the location of eachfrequency source must be unique, the resulting code is likewise uniqueeven if one or more of the transmitted frequencies is identical.

It is also known to use magnetic resonance (MR) for identification. Suchis the case, for example, in the field of Magnetic Resonance Imaging(MRI) or Nuclear Magnetic Resonance spectroscopy (NMR spectroscopy).Furthermore, it is possible to identify multiple resonant frequenciesdetected in parallel. For example, according to U.S. Pat. No. 5,341,099(“Magnetic resonance imaging apparatus”, assigned to Kabushiki KaishaToshiba, published on 1994) data is produced from asymmetrical echodata, and an MR image is reconstructed from the data thus produced.

Non-resonant frequencies, such as acoustical signals, are also used inthe art for encoding data. For example, U.S. Pat. No. 6,611,798(“Perceptually improved encoding of acoustic signals”, assigned toTelefonaktiebolaget LM Ericsson, published 2003) discloses encoding anacoustic source signal such that a signal reconstructed from the encodedinformation has a perceptually high sound quality. The acoustic sourcesignal is encoded into at least one basic coded signal that representsperceptually significant characteristics of the acoustic signal. Theencoder can include at least one spectral smoothing unit that receivesat least one of the signal components on which the basic coded signal isbased and generates in response thereto a corresponding smoothed signalcomponent.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative encodingmethod and system in which no predetermined spatial relationship betweenreader and tag is required.

This object is realized in accordance with a broad aspect of theinvention by a method for encoding a tag with an n-bit binary code(n→1), the method comprising:

a. associating with the tag one or more predetermined frequency sourcesthat produce known different respective characteristic frequencies; and

b. associating with each of said characteristic frequencies a knownunique position in the n-bit binary code.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 shows schematically a prior art tag that is encoded with multiplefrequency sources that are spatially constrained to generate a uniquecode;

FIG. 2 shows schematically a tag according to the invention that isencoded with multiple frequency sources that are spatially unconstrainedto generate a unique code;

FIG. 3 is a schematic representation of a tag-reader combinationconfigured to encode a 3-bit code;

FIG. 4 is a flow chart showing the principal operations carried out byan encoder according to the invention;

FIG. 5 is a flow chart showing the principal operations carried out by adecoder according to the invention;

FIG. 6 is a flow chart showing the principal operations carried out by astock management system according to the invention;

FIG. 7 is a flow chart showing the principal operations carried out by alo surgical instrument tracking system according to the invention;

FIG. 8 is a flow chart showing the principal operations carried out by aauthorization system according to the invention;

FIG. 9 is a block diagram showing functionally an encoder according tothe invention; and

FIG. 10 is a block diagram showing functionally a decoder according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2 there is shown a tag 20 according to the inventionthat is encoded with multiple frequency sources to generate an m-bitcode that is identified by a reader 21. By way of example only, thefollowing description will relate to a 9-bit code, although theprinciples of the invention can readily be applied for any integer valueof n greater than one.

Thus, for a 9-bit code the reader 21 pre-assigns 9 different frequenciesdesignated f₁, f₂ . . . f₉ to a unique respective position in the 9-bitcode. Thus, the first frequency source f₁ is assigned to the leastsignificant bit (LSB), while the frequency source f₉ is assigned to themost significant bit (MSB), each of the other seven bits of the 9-bitcode being assigned to corresponding frequencies f₁ to f₈. The tag 20 isencoded by providing only those frequency sources that are reflected inthe desired code. Thus, to encode 110101101 only the frequency sourcesf₁, f₂, f₄, f₆, f₈, and f₉ are required (assuming positive logic isused). It will be appreciated that negative logic could be employedinstead, whereby the absence of a frequency gives rise to logic “1” atthe corresponding location of the code.

In either case, the frequency sources f₁, f₂, f₄, f₆, f₈, and f₉ arespatially unconstrained. That is to say, neither the order in which theyappear in the tag nor their relative spatial location is significant.Indeed, all the frequency sources could be spatially superimposed one ontop of the other, so long as the reader is able to discern the presenceof all those frequency sources that are present. It is thus seen that,unlike the prior art system depicted in FIG. 1, in the invention aunique code is obtained merely by encoding the tag with a plurality ofpredefined frequency sources that produce known different respectivecharacteristic frequencies, each of which is associated with a knownunique position in the n-bit binary code.

Frequency sources may be used more than once, although multipleinstances of the same frequency source have no significance since thesame characteristic frequency is detected by the reader and affects onlya single bit of the n-bit code.

It should be noted that the frequency sources composing a tag are notnecessarily dispersed within a volume. Very small frequency sources canbe, for example, mixed in ink and printed on the identified object, suchas printing on paper. According to a different example, by mixing thefrequency sources with pigments used to color fabrics it is possible toprint a hidden tag on fabric (if the pigment used has the same color asthe fabric). It should be noted that any number of frequency sources maybe printed in this manner.

Likewise, a dispersion or colloid can be formed of different frequencysources, such as ferromagnetic elements and the resulting dispersion orcolloid then serves as a tag that encodes a unique code depending onwhich frequency sources are present.

It should also be noted that the characteristic frequencies of thefrequency sources need not be resonant or even particularly sharp. Thus,each frequency may be realized by a frequency band provided that therange of frequencies within each band can be distinguished from those ofan adjacent frequency band even if not from each other, thus allowingeach frequency band to be unambiguously detected. In this case, a seriesof band-pass filters may be used so as to pass all frequencies in agiven band, while blocking all other frequencies, thereby allowing eachfrequency band to be unambiguously detected.

FIG. 3 is a schematic representation of a tag 30 and a reader 31 thatare combined to encode and decode a 3-bit code that is characterized bythe presence or absence of three different frequency sources denoted f₁f₂ and f₃, which encode respectively the LSB, the second bit and theMSB. The reader 31 detects characteristic frequencies emitted by the tag30 and compiles a 3-bit code according to the following logic.

If all three frequency sources are absent from the tag, the combinationgenerated is 000;

-   -   If f₁ exists, and the other two frequency sources are absent,        the combination generated is 001;    -   If f₂ exists, and the other two frequency sources are absent,        the combination generated is 010;    -   If f₁ and f₂ exist, while is f₃ absent, the combination        generated is 011;    -   If f₃ exists, and the other two frequency sources are absent,        the combination generated is 100;    -   If f₁ and f₃ exist, while f₂ is absent, the combination        generated is 101;    -   If f₂ and f₃ exist, while f₁ is absent, the combination        generated is 110;    -   When all three frequency sources exist in the tag, the        combination generated is 111.

From the foregoing, it will be appreciated that the received frequenciesserve to identify the tag with which it is associated. The corollary isalso true in that on decoding an n-bit binary string corresponding to atag, it is possible to infer if a certain frequency source is associatedwith the tag or not. Thus, if in n-bit binary string according to theexample the third bit from the left is 1, this means that the stringencodes a tag with which the frequency source f₃ is associated.

When it is desired to increase the number of possible codes that can beencoded with the tag, one or more new bits must be allocated and thisimplies providing one or more new frequency sources each having arespective unique characteristic resonant frequency and assigning eachof the new f₁-frequency sources to known unique positions in the binarystring. For example, to expand the 3-bit code described above withreference to FIG. 3 of the drawings to a 4-bit code, the MSB of the4-bit code is associated with an additional, fourth frequency source f₄.

In general, to introduce a frequency source f₃ there should be allocateda bit representing f_(n)'s existence or absence to a known uniqueposition in the n-bit code.

FIG. 4 is a flow chart showing the principal operations carried out byan encoder according to the invention. Each bit is encoded successivelystarting with the LSB corresponding to M=1. If the corresponding bit isto be set to 1, then the frequency source corresponding to this bit isdeposited on the tag. The value of M is then incremented and the processrepeated in respect of successive bits in the n-bit code.

FIG. 5 is a flow chart showing the principal operations carried out by adecoder according to the invention. Each bit is decoded successivelystarting with the LSB corresponding to M=1. If the frequencycorresponding to each bit is detected, then the corresponding bit is setto 1. The value of M is then incremented and the process repeated inrespect of successive bits in the n-bit code. It should be noted thatany number of frequency sources may be decoded in this manner.

It should be noted that the flow charts illustrated in FIGS. 4 and 5represent non-limiting exemplary ways of encoding and decoding an n-bitcode and variations will be apparent to those of average skill in theart. It will also be appreciated that the manner in which the decoderdetects the frequency sources deposited on the tag does not affect themanner in which the n-bit code is decoded. Thus, the decoder can beconfigured to selectively receive each characteristic frequencysequentially and to assign a corresponding value to the bit associatedtherewith. Alternatively, it may be configured to receive all thecharacteristic frequencies in parallel as a complex signal and toperform frequency separation on the complex signal received from thereader. In this case, the separated frequencies emitted by the tag arerecorded so as to compile a list of all unique characteristicfrequencies associated with the tag. Subsequent decoding may thencontinue as for sequential decoding. An example of an applicablefrequency separation technique is described in above-mentioned U.S. Pat.No. 5,341,099.

It should further be noted that in the embodiments described so far,frequency sources (f₁ f₂ . . . f_(n)) are associated with the tag tocompose the n-bit binary tag, and the reader 204 detects thecorresponding characteristic resonant frequencies generated by thefrequency sources. The frequency sources used can be active frequencysources. An active frequency source can be, for example, a resonantcircuit such as the circuit used in WID, activated by a battery forexample. However, passive frequency sources may also be used.

An example of a passive frequency source is a resonant circuit activatedby energy transmitted to it by the reader. Such resonant circuits arewell known in smart card technology and are described, for example, inU.S. Pat. No. 5,241,160 issued Aug. 31, 1993 entitled “A System andMethod for the Non-Contact Transmission of Data” and assigned to OnTrack Innovations Ltd. Another approach is to use resonant elements, ina way known to those versed in fields like Nuclear Magnetic Resonance(NMR) spectroscopy or Magnetic Resonance Imaging (MRI).

Resonant elements such as ferromagnetic elements have characteristicresonant frequencies, induced by exposing the resonant element to apredetermined excitation frequency. Therefore, a resonant element can beused as a passive frequency source in tags according to the invention.It should be noted that resonant elements can be of any physical state,such as liquid, gas or solid.

Those versed in the art will appreciate that the term “characteristicfrequencies” as used above can apply to any frequency range within theelectromagnetic spectrum, so long as they are capable of being uniquelydetected by the reader. For example, acoustic signals can be used too,or even a combination of different ranges. A tag conveyingcharacteristic frequencies of any range can be encoded according to theinvention by n-bit binary codes, as long as the characteristicfrequencies are unique (forming, therefore, unique characteristicfrequencies), are associated with known unique positions in the n-bitbinary code and can be uniquely detected.

Tags that can be encoded by n-bit binary codes according to thedescribed embodiments can be utilized in many applications such as forautomatic identification of objects. For example, in a clothing store itis possible to mark each product with a tag including a uniquecombination of frequency sources, identifying the products thereby. Ifthere are two identical shirts in the store, for example, each

of them can receive a unique tag. Because the encoding method enables2^(n) different n-bit binary codes, this means that with 30 differentfrequency sources, for example, it is possible to have more than tenmillion different tags, which is enough to uniquely tag every product inan average store. Larger numbers of frequency sources enable even muchhigher number of identifying tags, of course.

FIG. 6 is a flow chart showing the principal operations carried out by adecoder according to such an embodiment when used in an anti-shopliftingsystem, for alerting suspected theft, where products are uniquelyencoded using tags. If the tags include passive frequency sources, theanti-shoplifting system should include a transmitter that induces thepredetermined excitation frequencies, and a reader 204, that detects thecharacteristic resonant frequencies emitted by the tags. When the tagsare placed in the zone susceptible to the transmitter, they convey theresonant frequency, which is then detected by the reader. The reader (ora processor coupled thereto) then decodes the tag so as to generate acorresponding n-bit binary code.

Alternatively, the tags can include active frequency sources that do notrequire a transmitter in order to convey their characteristic resonantfrequencies.

Each product in the store carries a unique tag bearing a respectiven-bit binary code whereby it is possible to identify the product type,and even the unique item. Therefore, purchasing a certain item, it ispossible to decode the item's tag, generating its n-bit binary code. Then-bit binary code can be added to a list of items authorized to leavethe premises of the store.

In all exits of the premises frequency readers are positioned in a waythat the exit forms at least part of the zone of the reader. That waythe readers can decode tags passing in their zone. If the decoding n-bitbinary string cannot be located in the list of items authorized to leavethe premises, this is considered as a suspected theft.

It should be noted that this example is non-limiting, and otherembodiments will be readily apparent to those versed in the art. Forexample, instead of having a list of items that are authorized to leavethe premises, it is possible to compile a list of all the items in thepremises, removing the item from the list when authorizing to take itout (i.e., on receiving payment at the checkout). On exiting thepremises the contents of the user's shopping basket are decoded, and ifa code obtained therefrom corresponds to an article in the list ofitems, this raises a suspicion that the article is unauthorized to leavethe place. Yet another approach is to shield a valid tag so that itsresonating frequencies cannot be detected by the readers at the exit.

In connection with the example above, it will be appreciated thatassociating such tags with items in a store or a library, for example,allows use of the tags for automatic checkout, or for stock management.

FIG. 7 is a flow chart showing the principal operations carried out by adecoder according to another embodiment for keeping track of surgicalinstruments during medical procedures for improving safety in anoperating theater. It is a known problem that in operating theaterssurgical instruments and materials, such as gauze pads, forceps orscalpels, are sometimes left in the patient's body, causing considerabledamage to the patient. One way to overcome the risk is having a staffperson count the instruments before the doctor begins sewing, makingsure everything is accounted for. This procedure is liable to error andcan be automated by uniquely marking each item with a tag according tothe invention. On moving a surgical instrument to within a working areaof the operating theatre the n-bit binary code of every surgicalinstrument can be decoded and listed in a running list of all thesurgical instruments that are in the working area. When leaving theworking area, surgical instruments' n-bit binary codes are removed fromthe running list. Upon termination of the surgical procedure the list ischecked, ensuring that all surgical instruments are accounted for.

Items can also be pre-encoded. For example, it is possible to print atag according to the invention on each gauze pad. It is known thatfrequently several gauze pads are packed together in one pack. Whenpacking the gauze pads it is possible to manage a list of n-bit binarycodes associated with the packed gauze pads. The list itself can beidentified by a unique n-bit binary code encoding a tag that can beprinted, for example, on the package.

When a package marked with a tag is moved into the working area, itscode is detected, inserting all the codes of the included gauze padsinto the running list of all surgical instruments that are in theworking area. At the same time it is possible to install a frequencyreader near a trash bin, in a way that the trash bin is in the zone ofthe reader. Whenever a gauze pad is disposed of in the trash bin, itsn-bit binary code is removed from the running list.

FIG. 8 is a flow chart showing the principal operations carried out by adecoder according to yet another embodiment for conditionally enablingand disabling electrical appliances. In vehicles, for example,immobilizers are commonly used to disable ignition without a predefinedkey. Tags according to the invention can be used as such predefinedkeys. A reader in the vehicle is programmed to recognize one or moren-bit binary codes. An authorized driver should carry a correspondingtag associated with one or more predetermined frequency sources thatproduce known different respective characteristic frequencies and eachof which is associated with a known unique position in an n-bit binarycode. When the tag is brought to the vicinity of the decoder, it isdecoded to provide the corresponding n-bit binary code. The immobilizerthen determines whether the detected n-bit binary code corresponds toany of the recognized n-bit binary codes, and if so, allows ignition ofthe vehicle's engine, which otherwise remains immobilized.

Furthermore, in a company having a fleet of vehicles, for example, theIDs of all employees who are authorized to drive the company's vehiclescan be used as n-bit binary codes programmed into the readers of all thecompany's vehicles, thus ensuring that only authorized employees candrive them. It is also possible to compile a list monitoring parametersassociated with employee use of the company's vehicle. For example, sucha list enables management to track the identity of each employee drivinga vehicle, the date when the vehicle was used, the distance driven, andso on.

FIG. 9 is a block diagram showing functionally an encoder for encoding atag 40 with an n-bit binary code stored in a code repository 41. Afrequency source unit 42 is coupled to the code repository 41 and isresponsive to an n-bit binary code stored therein for depositing inassociation with the tag 40 one or more predetermined frequency sourcesthat produce known different respective characteristic frequencies andthat are associated with a known unique position in the n-bit binarycode.

FIG. 10 is a block diagram showing functionally a decoder 50 fordecoding a tag 51 having an n-bit binary code. The decoder comprises adetector 52 for detecting one or more different characteristicfrequencies each of which is associated with a known unique position inthe n-bit binary code. Coupled to the detector 52 is a decoding unit 53that substitutes at respective positions of the n-bit binary stringrespective binary values according to a presence or absence of therespective characteristic frequency associated with the respectiveposition in the n-bit binary code.

1. A method for decoding an n-bit binary code encoded by a tag having associated therewith one or more predetermined frequency sources each adapted to emit a respective characteristic frequency associated with a known unique position in the n-bit binary code, the method comprising: (a) detecting characteristic frequencies emitted by the tag; and (b) substituting at respective positions of said n-bit binary string respective binary values according to a presence or absence of the respective characteristic frequency associated with the respective position in the n-bit binary code.
 2. The method of claim 1, wherein at least some of the characteristic frequencies are resonant frequencies.
 3. The method of claim 1, wherein at least some of the characteristic frequencies constitute acoustic signals.
 4. The method of claim 1, including detecting at least some of the characteristic frequencies in parallel.
 5. The method of claim 1, including detecting at least some of the characteristic frequencies sequentially.
 6. A decoder for decoding a tag having an n-bit binary code, said decoder comprising: a decoding unit responsive to one or more different characteristic frequencies each of which is associated with a known unique position in the n-bit binary code and for substituting at respective positions of said n-bit binary code respective binary values according to a presence or absence of the respective characteristic frequency associated with the respective position in the n-bit binary code. 